Single-phase alternating current electric motors conventionally are provided with two windings on a stator core, inductively coupled to the rotor of the motor. The windings generally constitute a starting winding (of relatively small diameter wire) and a run winding (of relatively large diameter wire compared to the starting winding) angularly displaced from one another on the stator. In capacitor-start and capacitor-start/capacitor-run motors, a starting capacitor is connected in series with the starting winding and a switch. At motor start-up, the switch is closed and the capacitor, in conjunction with the starting winding, is selected to produce a leading current in the start winding which is approximately equal to and approximately 90.degree. displaced in phase from the lagging current in the main or run winding of the motor. Such arrangements produce high values of starting torque with lower starting current than are obtained with other types of split-phase phase motors.
Usually, the switch in a capacitor-start motor is a centrifugal or thermal switch connected in series with the capacitor and start winding across the input terminals. The run winding then is connected in parallel with this series-connected starting circuit. In such capacitor start motors, the starting condition is such that the instantaneous locked rotor current is high and the motor starting current demand factor also is high. As a consequence, such motors undergo relatively high operating temperatures and require some type of switch for disconnecting or opening the starting winding circuit after a pre-established rotational speed of the rotor is reached. Because the starting winding of such motors generally is of relatively small diameter wire, over-heating can, and frequently does occur. Such overheating results in a relatively limited life of the starting winding due to burn out, particularly under overload conditions of operation of the motor.
Applicant's above mentioned co-pending application is directed to capacitor-start/capacitor-run motors which do not employ switches in the starting circuit, but which instead utilizes two series-connected windings (of substantially the same diameter heavy wire) electrically phase displaced 90.degree. from one another on the stator core. One of these windings has a capacitor connected in parallel with it to form a parallel resonant circuit at the operating frequency of the motor. The motor of this co-pending application is a high efficiency motor which overcomes most of the disadvantages of the prior art capacitor-start/capacitor-run motors. One disadvantage, however, which is present in the motor of the above identified co-pending application is that the starting torque of the motor is relatively low. As a consequence, applications for the motor of applicant's co-pending application are primarily for situations which do not require a very high starting torque, such as pumps, blowers, machine tools, and many commercial and domestic appliances.
Applicant also has three patents directed to single phase motors of the capacitor-start type directed to starting control circuits for motors producing high starting torque. These patents are U.S. Pat. Nos. 3,036,255; 3,573,579; and 3,916,274. The '255 patent is directed to a capacitor motor using either a centrifugal or relay operated switch in the starting circuit to open the capacitor starting circuit to disconnect it and the start winding from the motor operation during normal load conditions of operation of the motor.
U.S. Pat. Nos. '579 and '274 both are directed to solid state motor starting control circuits which do not employ mechanical switches. As a consequence, arcing, which is associated with mechanical switches, and the inherent shortcomings of mechanical switches, such as centrifugal switches, are overcome by the solid state circuitry used in the starting control circuit of these two patents. These patents, like other prior art for capacitor-start motors, however, have a starting capacitor connected in series with a start winding and the switch; so that starting current is applied through the start winding only during the start-up portion of operation of the motor. Once the motor reaches or nears operating running speed, the solid state switch creates an open circuit condition in the starting circuit; and the starting winding is removed from further operation. Consequently, such a solid state motor starting control circuit functions in a manner similar to the mechanical switch circuits of the prior art to control the connection and disconnection of the starting winding from the power input terminals in accordance with the particular state of operation of the motor.
It is desirable to provide a motor which has the advantages of the parallel resonant motor of the above identified co-pending application and which further is capable of producing a high starting torque, to permit use of such a motor in applications where the motor of applicant's above identified co-pending application is not suitable.